In the field of radio communications employing spread spectrum modulation, which includes hybrid frequency hop with phase modulation, transmission is accomplished using symbols which are demodulated as coherent wholes, each frequency hop using a matched filtering scheme. Since coherence cannot be maintained between frequency hops, it is necessary to use a modulation technique which achieves the greatest processing efficiency for each symbol. One such modulation technique is called cyclic code shift keying (CCSK). This modulation technique employs a binary bit sequence, having low autocorrelation sidelobe properties, which is cyclicly shifted a number of times proportional to the data word being transmitted. For example, a 32 bit word may be cyclicly shifted from 0 to 2.sup.5 times to encode a 5 bit word. This CCSK symbol is then modulo-2 added to a PN spreading function having the same or integrally related, higher bit rate to provide transmission security. Finally, the spread symbol is minimum shift keyed (MSK) modulated onto a carrier for transmission.
The MSK process can be considered to be either a QPSK process with sinusoidally shaped base-band signals or QPSK with linear phase transitions which are either +90.degree. or -90.degree., or an FSK process with a frequency deviation equal to exactly .+-.1/4 of the bit rate. In the FSK process one system waveform is defined as having the upper frequency when there exists no change on a bit boundary and having the lower frequency when a transition occurs on a bit boundary.
With the CCSK symbol mixed with a PN spreading function, the net effect is that, when the PN signal has a data transition, the MSK waveform has a +90.degree. phase transition when it would have been -90.degree., or vise versa. Thus, every time the PN waveform has a transition, the net effect is a 180.degree. phase reversal of the remainder of the symbol effected by changing the sign of the next 90.degree. phase shift.
In the demodulator for a CCSK waveform, the straightforward method for demodulation of a spread spectrum encrypted waveform is to first strip the PN spreading function off and then demodulate the CCSK data symbol. Noting the FSK definition of the MSK waveform, it can be seen that when the PN sequence makes a transition, the spectral components are inverted for one clock cycle. One method for stripping away the PN sequence might therefore be to provide a means for spectrum inversion at each PN transition. Unfortunately, such an operation is extremely difficult or substantially impossible to perform with phase continuity, which is essential to avoid decorrelating the remainder of the symbol.
A second method for stripping away the PN sequence would be to phase modulate the incoming signal with the PN waveform processed to provide a smooth 180.degree. phase variation over the duration of one clock cycle for each PN transition. A problem arises however, in that this phase variation must be sometimes positive and sometimes negative depending on whether or not a CCSK symbol transition occurred. To examine this further, if for example the incoming signal makes a phase variation of +90.degree. and stripping away the PN sequence would have made that -90.degree. then a phase variation of -180.degree. should be added in to effect the change. If on the other hand the reverse situation occurred, the required phase variation would be +180.degree.. If the wrong phase correction is applied, a 270.degree. phase rotation will occur which, while it ends up at the right place, will not correlate correctly with the CCSK reference. Thus, it can be seen that any attempt to provide PN stripping prior to the correlator causes significant decorrelation of the CCSK symbol on approximately 50% of the PN transitions.